This dissertation uses field observations to understand how changes in sediment transport affect the geomorphic evolution of the coastal Trinity River in Texas. I focus on the main channel of the river along the transition from quasi-uniform flow conditions into gradually varying flow. To begin, I will use elevation data in conjunction with stratigraphic analysis to suggest that scroll bars represent levee deposition along the inner bank of the river channel (Chapter 2). Because their channel-ward edge marks the transition from bed-material transport processes that are associated with the point bar and the proximal overbank deposits of the floodplain, they can be considered the geomorphic edge of the active channel form. Chapters three and four then focus on changes within the active channel through a range of time scales. Separate measurements of inner and outer bank volumetric change and migration show that river banks are capable of migrating independently over short time scales, but migrate compensationally over longer periods. The fifth chapter discusses the observation of a new scale of self-organization in bed topography that occurred along the base of the channel during a historic flood in 2015 (which was captured by the time-lapse lidar used in chapters 3 and 4). Repeat depth surveys show that 12 km long wavelength trains of dunes, spread out over an average of 12 river bends, advected and deformed downstream throughout the 55 day flood. These bedform groups are interpreted as an expression of prolonged washout that occurred over an initially spatially variable bedform field. Remaining questions and suggestions for future research directions are provided in the final chapter. Altogether, this dissertation represents a body of work that spans multiple temporal and spatial scales, and provides the community with a better understanding of how gradually varied flow affects sediment transport and geomorphology. In addition, it provides a foundation for future research that can illuminate important details about fluvial processes in general.